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The tangential velocity is the speed of
the blades' rotation. It increases with
distance away from the hub, until it
finally becomes a tangent to the edge
of the disc, hence the name.
Combined with the downwash velocity,
you end up with a resultant
corresponding with the blade's actual
speed and path, or the relative wind
(although its name suggests
otherwise, the downwash
component moves upwards):
36 JAR Private Pilot Studies
In autorotation, the function of
downwash velocity is replaced by air
going up through the rotors and
creating a larger angle of attack.
The flight velocity is the reciprocal of
the relative airflow, made up of
downwash and tangential velocity
and movement of the machine
through the air.
Translation is the conversion from
hover to forward movement, where
the helicopter is supported by other
means than its own power, that is,
relative airflow. Translational lift is the
extra thrust you get from forward
movement, when the new airflow
enters the disc. The helicopter flies
better because you get more air
through per unit of time, which has
a lower induced velocity because it
hasn't had a chance to speed up just
before going through the rotor. As
tip vortices are also being left
behind, your lift vector becomes
more vertical, for more thrust with
less drag. The reason you have to
lower the collective to maintain
height at this point is because the
angle of attack has increased against
the new relative airflow. This also
means less engine power is required.
Of course, all the while rotor
efficiency is increased with forward
flight, at some point you need to
increase power to overcome drag
from the fuselage, which is
increasing at a faster rate. This is
why you should not reduce power at
the end of a climb until you
have both the speed and height
you want (if you reduced
power at, say, 1500 feet and 60
knots, but you really wanted
100 knots, you wouldn't be able to
accelerate beyond a certain point
without applying more power than
you would have used before.
All changes in velocity from cyclic
movements are known as transitions.
Autorotations
Loss of RPM at the entry into
autorotation is the most significant
problem—a higher angle of attack
from the new relative airflow as air
rushes up through the rotors will
cause enough drag to slow the rotors
drastically, especially if your weight is
high or the air density low, meaning
that your blades will be at a higher
pitch angle anyway, to meet the
conditions of flight.
Reducing pitch to compensate will,
of course, increase the rate of
descent, at which point the inner
25% of each blade is stalled, and the
outer 30% is providing a small drag
force (in other words, it is being
driven). The best lift/drag ratio in
autorotation is at best endurance
speed (check the manual, but most
helicopters use about 45 kts). This is
when the driving region of the disc is
exactly centred. As you increase
speed, it moves toward the retreating
blade side of the disc until it touches
the edge, which is your power-off
VNE. If it goes beyond the edge, the
surface area of the driving region is
reduced, resulting in rotor decay:
Principles of Flight 37
When about 70 feet from the
ground, (depending on whether you
think you are descending or moving
across the ground too fast), use
rearward cyclic to slow down
vertically and horizontally, in a
manoeuvre called the flare. The
amount is proportional to your
speed and serves to increase the total
lift reaction (which stops the sink)
and shifts it to the rear (which stops
forward movement). It also increases
the rotor RPM. Continue the flare
progressively (and sharply), to be at
the correct speed for landing at 10
feet, applying collective as flare
effect decreases to check the descent
more positively, watching for drift
(the “check” is the application of
some collective to brake the descent
– in the 206 it can be a positive
movement; in the 407 and Astar, it
can just be a pause). As the flare
ends, and the kinetic energy of the
rotors is used when the collective is
raised, the airflow through the rotors
is reversed, assisting you to level (the
amount depending on the model),
ready to cushion the landing as you
apply collective pitch. This is where
correct use of airspeed during the
　
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